1. Field of the Invention
The present invention concerns a control method and circuitry for regulating a flow amount of a pressurized fluid by expansion and contraction of a fluid which fills a chamber, by means of a heat generating body, and by displacement thereby of a membrane constituting part of the chamber.
2. Description of the Related Art
FIG. 4 shows a valve device 10 which is used for controlling a flow amount of a pressurized fluid, such as compressed air, in an electrical signal--air pressure conversion apparatus or the like. A first wafer 16, formed by a single crystal silicon or similar glass material, is disposed inside of a casing 14 affixed to a substrate 12 in the valve device 10, and a hole 18 which communicates with an inlet conduit 20 is defined in a lower part of the first wafer 16. A nozzle 22 is formed at an opening of the hole 18 in the first wafer 16, wherein a small diameter discharge hole 24 is defined in the nozzle 22. Around the periphery of the nozzle 22, a discharge chamber 26 is defined.
A second wafer 30 is affixed to an upper surface of the first wafer 16. A chamber 34, having a silicon fluid or the like filled therein, which expands by application of applied heat thereto, is defined in the second wafer 30. A thin membrane 36 is formed on a bottom of the chamber 34, the membrane 36 being disposed at a predetermined interval separation away from the nozzle 22.
A glass wafer 38 formed of a heat resistant glass is affixed to an upper surface of the second wafer 30, wherein as a result, the chamber 34 is blocked and sealed in a fluid tight manner by the glass wafer 38. On a lower part of the glass wafer 38 which constitutes an upper surface of the chamber 34, a patterned heater 40 (heat generating body) is disposed, the heater 40 being connected to a circuit element on top of the substrate 12 via unillustrated electrodes and leads. A passage 42 communicating with the discharge chamber 26 is formed in the second wafer 30 and the glass wafer 38, and a discharge conduit 44 which is disposed in the casing 14 communicates with the passage 42.
A circuit 46 for controlling the valve device 10 formed in this manner is disposed on top of the substrate 12 which, as shown in FIG. 5, forms a bridge circuit 54 made up from the heater 40 and resistors 48, 50, 52, wherein a connection point 60 of the heater 40 and resistance 48, and a connection point 62 of resistors 50 and 52, are connected to an unillustrated temperature compensation circuit. A drive transistor 56 is connected to the bridge circuit 54, with a base 56 of the drive transistor 56 being connected to a command signal generator 58.
The control method of the valve device 10 shall now be described. When an analog voltage signal which serves as a command signal from the command signal generator 58, and corresponding to a desired compressed air flow amount, is input to the base of the drive transistor 56, a current corresponding to the analog voltage signal flows from the collector of the drive transistor 56 to the heater 40 of the valve device 10. As a result, the fluid 32 becomes heated by the heater 40 and the fluid 32 expands, whereby the membrane 36 is pressed by the expanded fluid 32 and approaches the nozzle 22. Accordingly, the flow of compressed air which is outlet from the discharge hole 24 into the discharge chamber 26 is restricted, and the flow amount thereof is reduced.
As the analog voltage signal changes and the flow of current to the heater 40 is reduced, the temperature of the fluid 32 decreases and the fluid 32 contracts, whereby the membrane 36 separates away from the nozzle 22. Thus, the flow amount of compressed air outlet to the discharge chamber 26 from the discharge hole 24 increases.
If the temperature of the heater 40 becomes high and the fluid expands excessively, there is a concern that the valve device 10 may become damaged. At that point, a change in the resistance amount of the heaters is output to an unillustrated temperature compensation circuit as a voltage difference of connection points 60 and 62 of the bridge circuit 54. A command signal output from the command signal generator 58 is controlled by the temperature compensation circuit, and the current conducted to the heater 40 is restricted, wherein the temperature of the heater 40 is reduced. In this way, damage to the valve device 10 can be prevented.
Notwithstanding, in the control method and circuit of the valve device 10 according to the above conventional technique, because the drive transistor 56 is driven by an analog signal, the electrical power consumed by the drive transistor 56 is large, and further, because the heat generated by the drive transistor 56 is large, when such heat is conveyed from the transistor 56 to the valve device 10, the fluid 32 becomes heated thereby and expands, with the problem that the flow amount characteristics of the compressed air are changed.